CFRP composite layups are becoming increasingly common in a wide variety of industries to construct component parts and structures that require superior strength-to-weight and strength-to-stiffness ratios. For example, in the aerospace industry, CFRP composite layups are constructed to form aircraft components (such as frames, ribs, spars, stringers and panels), which are used to manufacture aircraft structures (such as fuselages, wings, wing boxes, fuel tanks and tail assemblies), because of the significant weight reduction that such CFRP composite layups provide, the high structural rigidity and strength they provide, and the resulting airplane performance benefits.
CFRP composite layups generally comprise one or more composite layers or plies, each of the composite layers or plies are comprised of at least a reinforcement material and a matrix material. The matrix material is generally a non-conductive polymer such as an epoxy thermoset resin or a thermoplastic fluoropolymer that surrounds, binds and supports the reinforcement material, and transfers component stresses between layers of the reinforcement material. The reinforcement material in each layer or ply generally consists of strands of carbon fiber, which are electrically conductive and provide structural strength to the matrix material and the CFRP composite layups. During typical lay-up processes, one or more composite layers or plies are formed and/or placed (i.e., laid up) into a lay-up mold, mandrel or tool having a desired shape and size of the CFRP composite layup, and then cured and cut or trimmed to form the desired shape and size of the CFRP composite layup.
Cutting or trimming cured CFRP composite layups typically results in tips or ends of the conductive carbon fiber reinforcement material in each of the composite layers or plies becoming exposed to the environment at the cut or trimmed edges. Tips or ends of the conductive carbon fiber reinforcement material may also become exposed to the environment at external drop offs or sloped surfaces of a CFRP composite layup. For example, CFRP composite layups may be formed by laying up composite layers or plies with progressively shorter lengths on top of each other, such that the CFRP composite layups have a first end with a first length and a second end with a second length shorter than the first length, and a transition area between the first end and the second end with a sloped upper surface having a downward slope or drop off forming a non-vertical edge. Such drop offs may be positioned at any location along the length of a CFRP composite layup. The ends of the composite layers or plies forming the drop off may not be completely covered by matrix material, leaving the tips or ends of the conductive carbon fiber reinforcement material exposed to the environment.
Despite the presence of conductive carbon fibers in each of the composite plies, CFRP composite layups are not as electrically conductive as typical aerospace metallic structures and generally have more resistance to electricity, particularly in the z-direction (through the thickness of or perpendicular to the layup), than metallic structural materials such as aluminum, which is traditionally used, for example, in the aerospace industry. Thus, CFRP composite layups do not easily conduct and dissipate current associated with a lightning threat environment of commercial transport structures as described in SAE Aerospace Recommended Practice (ARP) ARP5414A, Aircraft Lightning Zoning, reaffirmed Sep. 28, 2012, and published by SAE International.
Among the physical phenomena observed from lightning strikes is a phenomenon known as “edge glow,” which describes the condition in which a glow of light possibly combined with particle or plasma ejections appears at the tips or ends of carbon fibers in exposed fiber surfaces of CFRP composite layups. As used herein, the term “exposed fiber surfaces” refers to any edges of a CFRP composite layup having exposed conductive carbon fibers, including cut or trimmed edges and edges in drop offs that are not completely covered by matrix material, as described above. Edge glow is caused by voltage differences between composite layers of the CFRP composite layups, and typically occurs in high current density areas resulting from a lightning strike, where the voltage potential difference between layers is at its maximum, such as at the exposed fiber surfaces.
The general design approach in the aerospace industry to prevent any possible negative effects of edge glow is to mask or seal all exposed fiber surfaces in any susceptible environment. One method is to seal the exposed fiber surfaces with a non-conductive or insulating material, such as a polysulfide sealant, that inhibits transmission of the glow of light and/or physically ejected particles. Another known method to guard against edge glow, sparking and plasma discharges around fastened joints in a composite structure and exposed fiber surfaces of a CFRP composite layup is to place an insulating, premolded cap over the exposed fiber surfaces or fastened joints.
The foregoing methods only mask or shield edge glow at the exposed fiber surfaces and do not prevent, reduce or eliminate edge glow. Also, these methods are labor intensive and require excessive production flow time because of the amount of work needed to prepare exposed fiber surfaces for successful sealant adhesion and the long cure times associated with the sealants. These methods also add significant weight to a composite structure made from CFRP composite layup components because of the required thickness and/or multiple layers of the sealants, which in the aerospace industry, increases aircraft fuel consumption and reduces performance.
It is therefore desired to make CFRP composite layups with less labor intensive and time consuming methods and with improved control of electrical current flow paths through the CFRP composite layup to mitigate edge glow and provide other electrical benefits with minimal weight penalty. As used herein, the term “mitigating edge glow” means reducing the intensity of edge glow or eliminating edge glow.